Top-down methods to produce micro and nanoparticles require the division of a macroscopic (i.e. millimetric) piece of matter, generally a liquid, into tiny offsprings of micro or nanometric size. Surface tension strongly opposes the huge increase of area inherent to this dividing process. Thus, to produce such small particles, energy must be properly supplied to the interface. This energy is the result of a mechanical work done on the interface by any external force field, i.e., hydrodynamic forces, electrical forces, etc. Two kinds of approaches can be distinguished, depending on how the energy is supplied.
In one approach, such as in the mechanical emulsification techniques, the force fields (extensional and shear flows) employed to break up the interface between two immiscible liquids are so inhomogeneous that, in general, the offspring droplets present a very broad size distribution. Nevertheless, a high degree of monodispersity might be achieved for a particular combination of the emulsification parameters (shear rate, rotation speeds, temperature, etc.) and a given combination of substances. However, such a desirable condition might not exist if one of the substances is changed, if a new one is added, or if a different size is desired. The same occurs if capsules must be formed. Furthermore, in many instances, the formation of the structure depends on chemical interactions, usually preventing the process from being applicable to a broad combination of substances.
In the other approach, which has the advantage of being based on purely physical mechanisms, the forces steadily and smoothly stretch the fluid interface without breaking it until at least one of its radii of curvature reaches a well-defined micro or nanoscopic dimension d; at this point, the spontaneous breakup of the stretched interface by capillary instabilities yields monodisperse particles with a size of the order of d. These types of flows are known as capillary flows due to the paramount role of the surface tension. For example, the formation and control of single and coaxial jets with diameters in the micrometer/nanometer range, and its eventual varicose breakup, lead to particles without structure (single jets) or compound droplets (coaxial jets), with the outer liquid encapsulating the inner one. On the other hand, if the liquid solidifies before the jet breaks, one obtains fibers (single jet), or coaxial/hollow nanofibers (coaxial jets). The mean size of the particles obtained with these methods ranges from hundreds of micrometers to several nanometers, although the nanometric range is generally reached when electric fields are employed. The particles obtained using this approach are, in general, nearly monodisperse and its employment enables, in the case of capsules, a precise tailoring of both the capsule size and the shell thickness. All these features make this approach particularly attractive for many technological applications.
Capillary flows capable of stretching out one or more interfaces up to the micro or submicron dimension have been the subject of considerable research, both experimental and theoretical, in the past few years. Although the Reynolds numbers of these capillary flows is of the order unity and smaller, the numerical simulation of some of them is complex due to (a) the disparity of length scales, which can vary more than three orders of magnitude, (b) the existence of a free surface that must be consistently determined from the solution of the problem, and (c) the fact that the region where the interface breaks is time dependent in spite of the steady character of the flow upstream of the breaking zone.
1.—Different Methods for Stretching Out Fluid Interfaces
In general, there are two ways to stretch fluid interfaces down to micrometric or sub-micrometric dimensions (A. Barrero and I. G. Loscertales, Micro and nanoparticles via capillary flows, Annual Rev. Fluid Mechanics, 39, 89-106, 2007). The first forces a liquid through an opening in a solid wall with characteristic dimension d and brings the curvature of the interface to that size; for example, forcing a fluid through a pipe or through a membrane with characteristic diameter or pore size d. For practical purposes, however, such small apertures are prone to clog for sizes below a few microns. The second approach uses suitable force fields instead of walls, to bring the curvature of the interface down to a scale d, much smaller than any boundary dimension. These forces are, generally, surface tension and fluid dynamic forces (pressure, inertia, and viscosity), although electrical and magnetic forces can also be used when the fluid reacts under these fields.
(A) Flows Through Micron-Size Apertures
A simple example of these flows is the injection of a fluid of density ρ and viscosity μ through a needle of micrometric diameter d immersed in an immiscible host fluid of density ρo and viscosity μo. The host fluid, which can also be a vacuum, may either be at rest or in motion with respect to the needle. At the end of the needle the interface between the two media evolves, governed by the following dimensionless parameters: the Weber and the Capillary numbers based on the characteristic velocity v of the injected fluid and on the interfacial tension γ between the two fluids, We=ρv2d/γ and Ca=μv/γ respectively, the Reynolds numbers of the host fluid flow based on its characteristic velocity vo, Reo=ρovod/μ, the viscosity and density ratios between the host and the injected fluids, μ and ρ, and finally, the angle α between the direction of vo and the needle axis. For a pair of fluids and a given geometrical configuration (given values of μ, ρ and α), the flow is governed by We, Ca, and Reo, which may vary in a broad range of values, thus giving rise to a very rich diversity of flows generally classified as dripping and jetting modes, which are respectively shown in FIGS. 1A and 1B.
The formation of jets and drops (or bubbles) at the end of a tube, and the transition from jetting to dripping, has been the subject of numerous investigations (O. A. Basaran, Small-scale free surface flows with breakup: Drop formation and emerging applications, AlChE J. 48, 1842-48, 2002; C. Clanet C & J. C. Lasheras. Transition from dripping to jetting. J. Fluid Mech. 383, 307-326, 1999). Droplets generated in the dripping mode are generally more monodisperse than those generated in the jetting mode. In particular, Umbanhowar et al. (2000) reported a method to produce nearly monodisperse emulsions (standard deviation less than 3%) that consists of detaching droplets from a capillary tip (α=0) in the presence of a coflowing stream (P. B. Umbanhowar, V. Prasad, D. A. Weitz, Monodisperse emulsion generation via drop break off in a coflowing stream. Langmuir 16, 347-351, 2000). The host fluid drags the meniscus formed at the end of the tip and detaches it to generate a drop with a diameter of the order of d. The coflowing method has been also exploited to generate highly monodisperse micron-size droplets of nematic liquid crystals to form two-dimensional (2D) and three-dimensional (3D) arrays for electro-optical applications (D. Rudhardt, A. Fernandez-Nieves, D. R. Link, D. A. Weitz, Phase-switching of ordered arrays of liquid crystal emulsions, Appl. Phys. Lett. 82, 2610, 2003; A. Fernández-Nieves, D. R. Link, D. Rudhardt, D. A. Weitz, Electro-optics of bipolar nematic liquid crystal droplets. Phys. Rev. Lett. 92, 05503, 2004; A. Fernandez-Nieves, D. R. Link, D. A. Weitz, Polarization dependent Bragg diffraction and electro-optic switching of three-dimensional assemblies of nematic liquid crystal droplets, Appl. Phys. Lett. 88, 121911, 2006).
The fact that the droplet pinch-off occurs at distances of the order of d (i.e., dripping) from where the needle ends severely narrows the break-up wavelength range. The needle diameter d acts as a wave filter, efficiently killing those wavelengths slightly away from a dominant one, which is of the order of d. This filtering effect is responsible for the extremely narrow size spectrum of the detached droplets. For the jetting mode, however, the pinch off occurs at a distance much larger than d from the needle, allowing the break-up wavelength range to broaden. Nonetheless, relatively monodisperse droplets are still obtained from these jets because the perturbation growth rate versus the perturbation wavelength usually exhibits a sharp maximum.
Other extensional flows within micron-sized channels have been also used to break single droplets in two daughter droplets whose size may be precisely controlled (D. R. Link, S. L. Anna, D. A. Weitz, H. A. Stone, Geometrically mediated breakup of drops in microfluidic devices. Phys. Rev. Lett. 92, 054503, 2004). In this implementation, an emulsion of micron-sized droplets continuously flows across a T-junction; the pressure-driven extensional flow splits the droplets in two, and each daughter droplet flows along each branch of the T.
(B) Micro-Flows Driven by Hydrodynamic Focusing
In micro-flows driven by hydrodynamic focusing, the interface between two fluids is stretched out by a highly accelerated converging motion of one of them that sucks the other one toward the converging point. One of the earliest implementations of this type of flow is the so-called selective withdrawal procedure. The first studies of this date back to the end of the 1940s (A. Craya, Recherches theoretiques sur l'ecoulement de couches superposees de fluids de densités différentes. L'Huille Blanche 4, 44-55, 1949; W. R. Debler, Stratified flow into a line sink, J. Eng. Mech. Div., Proc. Am. Soc. Civil Eng. 85, 51-65, 1959). The technique was largely employed in the field of geophysical flows before Cohen et al. (2001) applied the technique to coat micro-particles (I. Cohen I, H. Li, J. L. Hougland, M. Mrksich, S. R. Nagel. Using selective withdrawal to coat microparticles. Science 292, 265-267, 2001). In its simpler version, shown in FIG. 2, the tip of a tube of diameter D is located at a height H above an interface separating two immiscible liquids. By applying a steady suction throughout the tube, the resulting converging flow of the lighter fluid (the focusing liquid in this case) sets the other liquid into motion. For sufficiently small values of the suction, only the lighter liquid is withdrawn throughout the tube: the hydrodynamic forces cannot overcome the capillary forces, and the deformed interface eventually comes to rest. An increase in the suction leads to a transition where the heavier liquid is also withdrawn in the form of a steady-state thin jet of diameter d co-flowing with the focusing liquid (the lighter one) d being much smaller than D. The capillary breakup of this jet gives rise to a stream of droplets with a mean diameter of the order of that of the jet. For a given pair of liquids and a given tube diameter, there are two controlling parameters: the pressure drop along the tube, Δp, which controls the flow rate Q through the tube, and the distance between the tube exit and the interface, H. For a given value of H, increasing Δp results in a thicker jet, whereas for a given Δp, increasing H results in a thinner jet. In terms of dimensionless parameters, the dimensionless jet diameter d/D depends on the Reynolds number Reo=ρo Q/(μoD) and H/D. Note that for a given H/D, no steady-state jet is formed unless the Reynolds number becomes larger than a critical value implying there is a critical flow rate inherent to this technique.
Another implementation of this type of flow is the so-called flow focusing procedure (A. Gañán-Calvo, Generation of steady liquid micro-threads and micron-sized sprays in gas streams, Phys. Rev. Lett. 80, 285, 1998; A. Barrero, A novel pneumatic technique to generate steady capillary microjets, J. Aerosol Sci. 30, 117-125, 1999), where a pressure drop Δp across a thin plate orifice of diameter D causes a converging motion of the focusing fluid. A second fluid is injected at a rate q through a tube of diameter Dt, whose end is located a distance H in front of the orifice, Dt˜H˜D. For a given value of H, and an appropriate range of values of both q and Δp, the interface at the end of the tube develops a cusp-like shape from whose vertex a very thin steady-state jet of diameter d is issued (see FIG. 3).
The jet and the focusing fluid coflow throughout the orifice. The jet eventually breaks up into a stream of droplets with a mean diameter of the order of d. In the relevant cases, the characteristic jet diameter is much smaller than the orifice diameter d<<D. We note that flow focusing can also be achieved in two-dimensions (J. B. Knight, A. Vishwanath, J. P. Brody, R. H. Austin, Hydrodynamic focusing on a silicon chip: mixing nanoliters in microseconds, Phys. Rev. Lett. 80, 3863-3866, 1998.).
Note that, as in the selective withdrawal procedure, for a given pair of liquids and a given value of H, there are two controlling parameters: the pressure drop across the orifice, Δp, which controls the flow rate Q of the focusing fluid, and the injected flow rate q of the focused fluid. For a given value of q, the increase of Δp results in a thinner jet, whereas for a given Δp, the increase of q results in a thicker one. The dimensionless diameter of the jet, d/D, is a function of the Weber numbers of the focused and focusing flows, ρq2/(D3γ) and D Δp/γ, respectively, the ratio between the Capillary to the Reynolds number of the two flows, μ=μ2/(ρDγ) and μ0=μo2/(ρDγ), the density ratio ρ of the two fluids and the geometrical dimensionless parameters, Dt/D and H/D. Clearly, for a given pair of fluids and a given geometry, the jet diameter only depends on the Weber numbers of the two flows. Furthermore, in many experimental cases where a liquid is extruded by a focusing fluid, the viscosities of both fluids play almost no role, and the phenomenon can be predicted by the simple Bernouilli law, d=8ρq2/(π2Δp).
As with selective withdrawal, for a given Δp there is a minimum flow rate qmin, below which no steady jet can be formed. For this qmin, the jet diameter reaches its minimum value dmin, which is approximately given by the condition in which the pressure drop Δp balances the surface tension γ/dmin; this yields dmin=γ/Δp. For the case in which the focusing fluid is a gas of density ρg, the maximum value of Δp is of the order of ρga2, where a is the characteristic sound velocity of the gas; thus, for typical values of the surface tension γ, one obtains dmin˜1 micron.
Note that for the flows considered in this section, the diameters of the tubes and of the orifice are usually much larger than the jet diameter of the focused fluid; therefore, the solid walls do not filter out any break-up wavelengths, and consequently the droplets formed present a broader size distribution than those obtained by the co-flowing method in the dripping regime, considered in Section A (Flows through micron-size apertures). Furthermore, there is a slight difference between the two implementations described in this section that might influence the size distributions of the resulting droplets based on the stability of the flow, because in the flow focusing procedure the discharge of the focusing flow into a quiescent fluid, just after crossing the orifice, forms a shear layer that is unstable and develops into turbulence. This might affect the breakup of the thin jet when it occurs at distances larger than D downstream from the orifice.
There have been successful experiments relevant in producing emulsions (S. L. Anna, N. Bontoux, N. A. Stone. Formation of dispersions using “flow focusing” in microchannels. Appl. Phys. Lett. 82, 364-367, 2003) and microfoams (J. M. Gordillo, Z. Cheng Z, A. M. Gañán-Calvo, M. Márquez, D. A. Weitz D A. A new device for the generation of microbubbles. Phys. Fluids 16, 2828-2834, 2004) using a flow-focusing geometry integrated into a planar microchannel device. Results by Anna et al. (2003) show that the drop size as a function of flow rates and flow rate ratios of the two liquids (the focusing and the focused ones) includes a regime where the drop size is comparable to the orifice width (dripping) and one (jetting) where drop size is dictated by the diameter of a thin focused thread so that drops much smaller than the orifice are formed.
(C) Micro and Nanoflows Driven by Electrical Forces
(i) Electrospray.
The interaction of an intense electrical field with the interface between a conducting liquid and a dielectric medium has been known to exist since William Gilbert (1600), who reported the formation of a conical meniscus when an electrified piece of amber was brought close enough to a water drop (W. Gilbert, De Magnete, 1600. Transl. P. F. Mottelay. Dover, UK. 1958). Interface deformation is caused by the force that the electrical field exerts on the net surface charge induced by the field itself. Experiments show that the interface reaches a motionless shape if the field strength is below a critical value, whereas for stronger fields the interface becomes conical, issuing mass and charge from the cone tip in the form of a thin jet of diameter d. In the latter case, the jet becomes steady if the mass and charge it emits are supplied to the meniscus at the same rate. Taylor (1964) explained the conical shape of the meniscus as a balance between electrostatic and surface tension stresses; since then the conical meniscus has been referred to as the Taylor cone (G. I. Taylor. Disintegration of water drops in an electric field. Proc. R. Soc. Lon. A 280, 383-397, 1964). The thin jet eventually breaks up into a stream of highly charged droplets with a diameter of the order of d. This electrohydrodynamic steady-state process is so-called steady cone-jet electrospray (M. Cloupeau, B. Prunet-Foch. Electrostatic spraying of liquids in cone-jet mode. J. Electrost. 22, 135-159, 1989), or just electrospray (C. Pantano, A. M. Gañán-Calvo, A. Barrero. Zeroth-order electrohydrostatic solution for electrospraying in cone jet mode. J. Aerosol Sci. 25, 1065-1077, 1994).
The electrospray has been applied for bioanalysis (J. B. Fenn, M. Mann, C. K. Meng, S. K. Wong, C. Whitehouse C. Electrospray ionization for mass spectrometry of large biomolecules. Science 246, 64-71, 1989), fine coatings (W. Siefert. Corona spray pyrolysis: a new coating technique with an extremely enhanced deposition efficiency. Thin Solid Films 120, 267-274, 1984), synthesis of powders (A. J. Rulison, R. C. Flagan. Synthesis of Yttrya powders by electrospray pyrolysis. J. Am. Ceramic Soc. 77, 3244-3250, 1994), and electrical propulsión (M. Martínez-Sánchez, J. Fernández de la Mora, V. Hruby, M. Gamero-Castaño M, V. Khayms. Research on colloidal thrusters. Proc. 26th Int. Electr. Propuls. Conf., Kitakyushu, Jpn., pp. 93-100. Electr. Rocket Propuls. Soc. 1999), among other technological applications. Recently, electrosprays in cone-jet mode were also performed inside dielectric liquid baths to produce fine emulsions (A. Barrero, J. M. Lóopez-Herrera, A. Boucard A, I. G. Loscertales, M. Marquez. Steady cone-jet electrosprays in liquid insulator baths. J. Colloid Interface Sci. 272, 104-8, 2004).
In electrosprays, a flow rate q of a liquid with electrical conductivity K is fed through a capillary tube of diameter Dt connected to an electrical potential V with respect to a grounded electrode. Given a liquid and a geometrical configuration of the tube-grounded electrode an electrospray forms at the end of the tube for a certain range of values of both q and V. Within this range, the effect of both the voltage V and the electrode geometry on either the current I transported by the jet or its diameter d is almost negligible for most experimental conditions, leaving the flow rate q as the main controlling parameter. Furthermore, the liquid viscosity μ affects only the jet breakup, but neither I nor d. For a given liquid one has: d=do f(β,q/qo) and I=Io g(β,q/qo), where do=[γ∈o2/(ρK2)]1/3, qo=γ∈o/(ρK), ∈o and β are the vacuum permittivity and the dielectric constant of the liquid respectively, and functions f and g must be experimentally determined.
Experimental and numerical studies on the scaling law of I have provided the widely accepted relationship, I=g(β)(γKq)1/2, with g(β)˜β1/4 (J. Fernández de la Mora & I. G. Loscertales. The current emitted by highly conducting Taylor cones. J. Fluid Mech. 260, 155-184, 1994; A. M. Gañán-Calvo, J. Dávila, A. Barrero. Current and droplet size in the electrospraying of liquids. Scaling laws. J. Aerosol Sci. 28, 249-275, 1997.)
However, the scaling law for the jet diameter d remains still controversial because experimental errors in the reported measurements of the mean droplet size do not allow the distinction between the different proposed size laws. The scaling size laws that appear most frequently in the literature can be cast in the form, d˜f(β) do(q/qo)n, where f(β)˜1 and n takes the values ⅓, ½, and ⅔, depending on the authors.
For electrosprays, experimental data and scaling laws show that the minimum jet diameter that can be achieved is of the order of one micron for liquids with electrical conductivities of the order of 10−3 S/m, but if K takes values of the order of 1 S/m, then dmin becomes of the order of 10 nanometers.
(ii) Electrospinning.
The electro-hydrodynamic flow described above can also be used to obtain very thin fibers if the jet solidifies before breaking into charged droplets. This process, known as electrospinning, occurs when the working fluid is a complex fluid, such as the melt of polymers of high molecular weight dissolved in a volatile solvent (J. Doshi & D. R. Reneker. Electrospinning process and applications of electrospun fibers. J. Electrost. 35, 151-160, 1995; S. V. Fridrikh, J. H. Yu, M. P. Brenner, G. C. Rutledge. Controlling the fiber diameter during electrospinning. Phys. Rev. Lett. 90, 144502, 2003). The rheological properties of these melts, sometimes enhanced by the solvent evaporation from the jet, slow down and even prevent the growth of varicose instabilities. As is well-known, large values of liquid viscosity delay the jet breakup by reducing the growth rate of axisymmetric perturbations, so longer jets may be obtained. However, non-symmetric perturbation modes can also grow due to the net charge carried by the jet. Indeed, if a small portion of the charged jet moves slightly off axis, the charge distributed along the rest of the jet will push that portion farther away from the axis, thus leading to a lateral instability known as whipping or bending instability. A picture capturing the development of the whipping instability in a jet of glycerin in a hexane bath is shown in FIG. 4.
The chaotic movement of the jet under this instability gives rise to very large tensile stresses, which lead to a dramatic jet thinning. The solidification process, and thus the production of micro- or nanofibers, is enhanced by the spectacular increase of the solvent evaporation rate due to the thinning process. For the production of nanofibers, this technique is very competitive with other existing ones (i.e., phase separation, self-assembly, and template synthesis, among others), and is therefore the subject of intense research.
(D) Steady-State Coaxial Capillary Flows for Core-Shell Micro and Nanoparticles
Micro and nanoparticles with a well-defined core-shell structure may also be obtained from flows obeying the same basic principles as those reviewed in the previous section; in this case, however, two interfaces separating three fluid media are required to produce the core-shell structure. The motion of the liquids must result in a coaxial stretching of the two interfaces and the breakup of the interfaces in this coaxial configuration may lead to core-shell particles. For instance, either core-shell capsules or fibers can be obtained from a coaxial jet, depending on whether the jet breaks or solidifies, respectively. These types of coaxial flows are governed by twice the number of parameters as those described previously, and so may exhibit many more regimes. However, when seeking the steady-state condition, the possible regimes are limited.
(i) Hydrodynamic Focusing in Fluidic Devices.
Utada et al. (2005) introduced a fluidic device based on hydrodynamic focusing that generates double emulsions in a single step in the micrometric range (A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stone, D. A. Weitz. Monodisperse double emulsions generated from a microcapillary device. Science 308, 537-54, 2005). In their device, sketched in FIG. 5, three immiscible fluids are forced through a converging exit orifice. The converging flow of the outer fluid stretches out the two interfaces between the fluid media whose breakup by capillary instabilities forms core shell drops.
In steady-state conditions, two operative regimes, dripping or jetting, may be established. Dripping produces drops close to the entrance of the collection tube within a single orifice diameter, analogous to a dripping faucet. In contrast, jetting produces a coaxial jet that extends three or more orifice diameters downstream into the collection tube, where it breaks into drops. For a given dripping condition, an increase of the flow rate of the focusing fluid (the outermost) beyond a threshold value causes the interface to abruptly lengthen, defining the transition to the jetting regime. Droplets produced by dripping are typically highly monodisperse, whereas the jetting regime typically results in polydisperse droplets whose radii are much greater than that of the jet. However, these authors discovered a very narrow window of operational conditions in which jetting yields a monodispersity similar to that of dripping. The size distribution of the double emulsions is determined by the break-up mechanism, whereas the number of innermost droplets (i.e., core-shell or multivesicles capsules) depends on the relative rates of drop formation of the inner and middle fluids. When the rates are equal, the annulus and core of the coaxial jet break simultaneously, generating a core-shell drop.
(ii) Electrified Coaxial Jet.
Particles with core-shell structure were recently obtained from electrified coaxial jets with diameters in the nanometer range (I. G. Loscertales, A. Barrero, I. Guerrero, R. Cortijo, M. Márquez, A. Ganan-Calvo. Micro/nano encapsulation via electrified coaxial liquid jets. Science 295, 1695-1698, 2002). In this technique, two immiscible liquids are injected at appropriate flow rates through two concentrically located capillary needles. At least one of the needles is connected to an electrical potential relative to a ground electrode. The needles are immersed in a dielectric host medium that may be gas, liquid, or vacuum. For a certain range of values of the electrical potential and flow rates, a compound Taylor cone is formed at the exit of the needles, with an outer meniscus surrounding the inner one (see FIG. 6a). A liquid thread is issued from the vertex of each one of the two menisci, giving rise to a compound jet of two co-flowing liquids (see FIG. 6b). To obtain this compound Taylor cone, at least one of the two liquids must be sufficiently conducting. Similarly to simple electrosprays, the electrical field pulls the induced net electric charge located at the interface between the conducting liquid and a dielectric medium and sets this interface into motion; because this interface drags the bulk fluids, it may be called the driving interface. The driving interface may be either the outermost or the innermost one; the latter happens when the outer liquid is a dielectric. When the driving interface is the outermost, it induces a motion in the outer liquid that drags the liquid-liquid interface. When the drag overcomes the liquid-liquid interfacial tension, a steady-state coaxial jet may be formed. On the other hand, when the driving interface is the innermost, its motion is simultaneously diffused to both liquids by viscosity, setting both in motion to form the coaxial jet.
Scaling laws showing the effect of the flow rates of both liquids on the current transported by these coaxial jets and on the size of the compound droplets were recently investigated (J. M. López-Herrera, A. Barrero, I. G. Loscertales, M. Márquez. Coaxial jets generated from electrified Taylor cones. Scaling laws. J. Aerosol Sci. 34, 535-552, 2003). This technique has been used to generate, upon coaxial jet breakup, core-shell micro- and nanocapsules and microemulsions (G. Larsen G, R. Velarde-Ortiz, K. Minchow, A. Barrero, I. G. Loscertales. A method for making inorganic and hybrid (organic/inorganic) fibers and vesicles with diameters in the submicrometer and micrometer range via sol-gel chemistry and electrically forced liquid jets. J. Am. Chem. Soc. 125:1154-55, 2003; I. G. Loscertales, A. Barrero, M. Márquez, R. Spretz, R. Velarde-Ortiz, G. Larsen. Electrically forced coaxial nanojets for one-step hollow nanofiber design. J. Am. Chem. Soc. 126, 5376-5377, 2004; A. Barrero, J. M. López-Herrera, A. Boucard, I. G. Loscertales, M. Márquez. Steady cone-jet electrosprays in liquid insulator baths. J. Colloid Interface Sci. 272, 104-108, 2005).
It is important to point out that the mean size of the capsules may be submicronic in contrast to the technique described in the previous section. On the other hand, the size distributions are broader than those obtained there; nonetheless, polydispersities of 10% can be obtained. Similarly to electrospinning, solidification of the outer liquid leads to hollow nanofibers (Loscertales et al. 2004; D. Li D, Y. Xia. Direct fabrication of composite and ceramic hollow nanofibers by electrospinning, Nano Lett. 4, 933-938, 2004; M. Lallave, J. Bedia, R. Ruiz-Rosas, J. Rodriguez-Mirasol, T. Cordero, J. C. Otero, M. Marquez, A. Barrero, I. G. Loscertales, Filled and hollow carbon nanofibers by coaxial electrospinning of Alcell lignin without binding polymers, Adv. Mat. 19, 4292, 2007), whereas solidification of the two liquids leads to coaxial nanofibers (Z. Sun, E. Zussman, A. L. Yarin, J. H. Wendorff, A. Greiner. Compound core-shell polymer nanofibers by co-electrospinning. Adv. Mater. 15, 1929-1932, 2003; J. H. Yu, S. V. Fridrikh, G. C. Rutledge. Production of submicrometer diameter fibers by two-fluid electrospinning. Adv. Mater. 16, 1562-66, 2004; J. E. Díaz, A. Barrero, M. Márquez, I. G. Loscertales. Controlled encapsulation of hydrophobic liquids in hydrophilic polymer nanofibers by electrospinning. Advanced Functional Materials, 16, 2110-2116, 2006). This process has been termed coelectrospinning.